1. Technical Field of the Invention
The present invention relates to the implementation of metal barriers in integrated electronic circuits.
2. Description of Related Art
Integrated circuits generally comprise such metal barriers at the interface between a conductive zone (typically a metal line or via in a multilayered interconnect structure) and an insulating zone (typically a dielectric layer in such a structure). These barriers prevent the diffusion of atoms from the conductive zone to the insulating zone, which can be the origin of integrated circuit malfunctions such as short-circuits between metal lines. This is why these barriers are commonly called “metal caps.”
Such metal barriers can also act as conductive passivation layers to mitigate the weak antioxidation properties of a conductive material such as copper (Cu).
These two functions, the antidiffusion and antioxidation functions, are combined.
Such metal barriers are generally realized of a material based on titanium (Ti) or tantalum (Ta).
It has also been proposed to use cobalt (Co), or metal compounds based on cobalt, as a material for realizing these metal barriers. In what follows, such compounds are denoted as Co-M, where the letter M indicates a metal which in particular can be tungsten (W), phosphorus (P), boron (B), or a combination of these metals. It is advantageous for such a material to be selectively deposited and self-aligned by an electroless process. In this process, the self-aligned barriers comprising cobalt are manufactured by placing the wafer in a solution. The solution reacts with the copper such that the barrier only forms on the surface of the copper. This type of process therefore only requires preparing the wafer surface and controlling certain parameters such as the temperature and pH.
In particular, an alloy of cobalt and tungsten-phosphorus (denoted Co-WP and called “cobalt-tungsten-phosphorus”) is known to be a good candidate for realizing self-aligned barriers (SAB). Co-WP improves the electromigration properties of the conductive zones of the interconnect structures in circuits using CMOS technology (Complementary Metal-Oxide Semiconductor), resulting in high performance circuits. It is also an economical solution for realizing CMOS imagers, with the added advantage of increasing the optical gain.
In practice, however, a cobalt-based metal barrier tends to oxidize, particularly when it is exposed to ambient oxidants during later steps involving the deposition of a dielectric material, the plasma etching of this material, etc., as the semiconductor fabrication process continues.
The effect of this undesired oxidation is illustrated in FIG. 1, which is a partial view of an interconnect structure of a semiconductor product with two metallization levels.
The first and second metallization levels of the interconnect structure, respectively labeled M1 and M2, are realized in respective layers of a dielectric material 10. This dielectric material can be of undoped silicon dioxide (SiO2) (USG, for “undoped silicon glass”), tetraethoxysilane (TEOS), fluorosilicate glass (FSG), BD2X™, etc. Level M1 comprises a metal line 11. Level M2 comprises a metal line 12 as well as a via 13 connecting the line 12 to the line 11 in the lower level. These metal elements 11, 12 and 13 are of copper for example, but they can also be of another metal such as aluminum (Al). Before the deposition of the metal constituting the line 11 (or the line 12 and the via 13), the walls of the corresponding trench in the dielectric material 10 of the level M1 (or the level M2 respectively) are spray coated with tantalum (Ta), with said tantalum then at least partially nitrided to result in a metal cap or antidiffusion barrier 14 (or 17 respectively) comprising tantalum nitride and tantalum (TaN/Ta). Similarly, a metal self-aligned barrier based on cobalt 15 is formed before the deposition of the dielectric material 10 of the level M2. During the deposition and etching of the dielectric material 10 of the level M2, a partial or complete oxidation of the Co-M of the layer 15 may occur such that a metal oxide layer 16 (Co-M-Ox) is formed on top of the layer 15. In extreme cases, the copper of the underlying conductive line 11 can also oxidize to form a layer 18 of copper dioxide (CuO2).
These undesired oxidations can result in:                an increase in the electrical resistance of the material forming the metal barrier, and therefore an increase in the electrical resistance of the vias,        a decrease in the antidiffusion properties of the metal barrier preventing the diffusion of copper atoms,        the formation of voids when wet cleaning is performed after plasma etching the via; such cleaning is well known to a person skilled in the art, and is intended to remove the polymer residues formed during the etching step, particularly at the edge of the vias; however, such cleaning may cause the removal of the Co-M, particularly if it is in oxidized form (Co-M-Ox), and create voids (labeled with the number 19 in the diagram in FIG. 2) which can result in integrity failure of the metal cap 17, appearing as a flaw in said cap (designated by the circle 20 in the diagram in FIG. 3), and        a decrease in performance in terms of electromigration.        
It is known that metal silicides have better antioxidation properties than their pure metal counterparts. For example, tungsten silicide (WSi2) is less sensitive to oxidation than tungsten. In addition, the metal silicide layer formed on the surface of the copper when a self-aligned barrier comprising silicon is formed, allows better retention of the copper atoms when a relatively high current is applied to the circuit, which means the resistance against electromigration is also improved.
Selective silicidation of pure cobalt for an antioxidation barrier in copper-based metallization is known, for example in the article “Selective Silicidation of Co Using Silane or Disilane for Anti-oxidation Barrier Layer in Cu Metallization”, S. Noda, R. Hirai, H. Komiyama, and Y. Shimogaki, Japanese Journal of Applied Physics, Vol. 43, No. 9A, pp. 6001-6007 (2004).
However, even after the incorporation of silicon atoms by a silicidation process, the cobalt-based metal compound Co-M continues to undergo some oxidation (although to a lesser extent) in the context described above, and the diffusion of copper atoms across this barrier is not completely prevented either.
This is why a need exists to further improve the antioxidation properties and the resistance to the diffusion of copper atoms in cobalt-based metal barriers used in the manufacture of certain integrated circuit structures.